This invention relates generally to a transmission system for transmitting data by utilizing a phase shift keying signal (hereinafter referred to as a PSK signal) which is obtained by phase-shift keying a carrier wave with a data signal and more particularly to a method and apparatus for demodulating the PSK signal to regenerate the data signal which are switable for realization of a PSK signal demodulator in the form of a digital integrated circuit.
A coherent detection scheme as diagrammatically shown in FIG. 1 is well known as a method for demodulating the PSK signal to regenerate the data signal. Digital data is differentially coded to obtain a signal a in FIG. 2 and a carrier is phase-shift keyed with the signal a to produce a PSK signal which is transmitted on a transmission line. The PSK signal is received by a band-pass filter 21 and converted by a slicer (limiter) 22 into a waveform b as shown in FIG. 2. The waveform b branches, on the one hand, to a carrier regenerator 23 which regenerates from the waveform b the carrier, designated at C in FIG. 2, serving as a reference phase and, on the other hand, to a phase comparator (coherent detector) 24 which performs coherent detection of the waveform b by using the regenerated carrier C. The output signal of the phase comparator 24 takes a waveform d which contains noise pulses due to jitter based on noise added to the signal as it propagates on the transmission line and a phase error of the regenerated carrier C. The noise pulses can be removed by means of a low-pass filter 25 and a slicer 26. Thus, the output signal of the slicer 26 takes a waveform e. A code converter 27 is adapted to decode the differentially coded data signal.
It will be seen from the above that the coherent detection scheme includes analog circuits such as carrier regenerator 23 and low-pass filter 25. Conventionally, some systems have been directed to replacement of the analog circuits with digital circuits suitable for integrated and adjustless form. For example, JP-A-No. 51-48959 and JP-A-No. 55-4125 disclose realization of the low-pass filter 25 with a digital circuit.
Thus, prior systems are dedicated to the realization of the low-pass filter 25 with a digital circuit, indispensably requiring the regeneration of the carrier. No realization of the carrier regeneration with a digital circuit can be found in any prior art devices. This is because the carrier regeneration is essentially, an analog operation which is carried out on the time axis. Accordingly, it has hitherto been difficult to achieve complete realization of the demodulator with a digital circuit.